Field of the Invention
The invention relates to an optical system and a display apparatus. More particularly, the invention relates to an illumination system and a projection apparatus.
Description of Related Art
With advances in display technology, there are a variety of display devices available for users to choose, such as liquid crystal displays, organic light-emitting diode (OLED) displays and projection devices. Since projection devices may produce larger images in smaller sizes, some applications thereof can not be replaced, such as presentations used for conference meeting, home theaters, classroom education or research, and so forth, in which people participate.
In recent years, projection devices equipped with light-emitting diodes (LEDs) or laser diodes as a solid-state light source gradually dominate the market, wherein the laser diodes gain much attention for its higher luminous efficiency. The projection devices equipped with laser diodes as a solid-state light source produce desired pure lights by exciting phosphors by the laser light emitted from a light source, and using a color wheel to achieve sequential displays and a purpose of enhancing purity of the desired color light.
Although laser diodes have higher luminous efficiency, such projection devices still face an issue of parts of color lights having insufficient brightness. In the prior art, brightness of color lights is enhanced primarily by adjusting compositions of phosphor materials; however, this method is not adapted for adjusting brightness of particular color lights. In addition, there are different methods for controlling light-emitting ratios of different color lights by adjusting areas of corresponding colors on the color wheel; however, this method may lead to other issues. Take red light as an example, red phosphor is rarely used as an excited material due to its poor reliability (low tolerance and poor conversion efficiency). Therefore, the red light is usually obtained by arranging phosphor capable of exciting yellow light in coordination with a red filtering sheet on the color wheel and allowing the excited yellow light to pass through the red filtering sheet. However, the red filtering sheet would filter out portions of the red light having wavelength with the yellow band. Consequently, such approach could not effectively enhance brightness of the red light. On another aspect, if an area of the red filtering sheet on the color wheel is adjusted for enhancing brightness of the red light, areas of other colored filtering sheets have to be inevitably adjusted. When the areas are not adjusted properly, the display devices may have issues of color shifts or insufficient brightness of parts of the color lights. Accordingly, how to effectively enhance brightness of different color lights and avoid color shifts have become one of urgent issues to be solved by researchers nowadays.
China Patent No. 103062672 discloses a projection apparatus provided with phosphors in different colors and color wheels to achieve full color. U.S. Pat. No. 8,469,520 discloses a projection apparatus for enhancing color rendering property by arranging reference light sources in different colors. U.S. Patent Publication No. 20130083509 discloses a projection apparatus for enhancing brightness of each color light by arranging reference light sources with different colors.